Catheters to be introduced into the human body have been known for many years. The first description of such a catheter for probing the heart is from W. Forssmann and appeared in Klinische Wochenschrift 8, 1929, pp. 2085 ff. A great variety of experience has been gathered with such catheters in the area of diagnostics. A survey of the catheters used and the diagnostic measures to be performed therewith can be found in the company publication Hewlett-Packard Application Note 762 "A Guide to Hemodynamic Monitoring using the Swan-Ganz Catheter", 1978. As of the seventies, catheters with so-called dilatation balloons have also been used. Such balloons are inflatable from the outside and serve to widen congenital or acquired narrowings (stenoses) in the human vascular system. See for example A. Cribier et al., "Percutaneous Transluminal Balloon Valvuloplasty of Adult Aortic Stenosis": Report of 92 cases, published in JACC Vol., No. 2, Feb. 1987, pp. 381 ff.
One diagnostic measure, for example, is to take hydrostatic pressure measurements with one catheter having a plurality of separate channels and a plurality of lumina, a so-called multilumen catheter. The individual channels are directed toward the exatracorporal proximal end of the catheter and may be connected via standardized connections to pressure sensors for measuring the hydrostatic pressures at the lumen openings of the individual channels and the pressure difference or pressure gradient. Such a catheter may also be provided additionally with a temperature sensor, e.g. a small thermistor, for taking measurements of the cardiac pumping behavior, i.e. measurements of the cardiac output, by a so-called thermodilution method. By this method, according to the prior art, a cryogen solution is injected from the proximal end of the catheter via a channel into the blood vessel, e.g. the right ventricle of the heart, which solution after a certain time flows past the temperature sensor located toward the distal end. The temperature drop at the temperature sensor location as a function of the flow time of the cryogen solution in the blood circulation can then be used to determine the cardiac output, as set forth for example in U. S. Pat. Nos. 4,502,488 and 4,105,022. The connections of the temperature sensor are directed in the catheter tube to the proximal end and are connected there via standardized connections with a microcomputer, which directly states the required measured values, such as the cardiac output in liters per minute. Such measurements up to now have been taken primarily only in the right heart, with the catheter being pushed through a vein into the right heart or the pulmonary circulation.
A typical case of routine application of such a right heart and pulmonary cardiac output measuring catheter is with a patient having a previous myocardial infarction or some type of cardiomyopathy and therefore reduced pumping function of the heart. In such a patient a typical procedure is to apply a left ventricular angiography catheter and to inject dye into the left ventricle and by means of X rays to determine the contraction pattern of this left ventricle. It is described as ejection fraction indicating how much of the percentage of the ventricular volume is ejected by one heart beat. In addition to this indicator of the overall ventricular function, routinely now a second catheter is put into the venous and right heart system of the patient in order to measure cardiac output by the conventional way described above to thereby determine the overall pumping function of the heart in liters of blood pumped per minute. Of special interest also is the knowledge of cardiac output in diagnosing the degree of severeness of valvular stenosis or regurgitation. The prior art required the use of two catheters, one in the right heart to measure cardiac output and one in the left heart to measure pressure and ejection fraction.
One condition for the measurement of heart output in the left heart by the thermal dilution effect is that the direction of blood flow and the direction of the injected cryogenic solution is opposite. This has been identified as a problem in U.S. Pat. No. 3,726,269 W. Webster, Jr. Apr. 10, 1973 to be solved by providing two different temperature sensors placed respectively in the blood flow and the cryogenic solution flow paths for measuring coolant and dilution temperatures separately, This technique requires one temperature sensor to be located within the catheter and the other to be located outside the catheter in the blood flow path being diluted by the cryogenic solution. Thus the design and placement of the two required temperature sensors critically restricts the location and construction of the catheter. In particular, for each temperature measurement this technique requires placement of separate wires to multiple temperature sensors, and special construction for extending wires through the catheter walls. Also determination of the significant temperature from which cardiac output may be determined with two such sensors is limited to the temperature at a single point in the blood flow path, namely the position of the sensor located outside the catheter. Accordingly even though there are two temperature sensors, the catheter would have to be moved about to determine the blood temperature at two significantly different positions in the cardiovascular system of the patient. This prior art temperature system is limited to the determination of the cardiac output and the ventricular volume. The latter critically relates to peak thermal amplitudes, a function of the injection rate of the cryogenic solution, which is usually manually controlled and thus not subject to precise rate control.
Also the prior art thermodilution technique presents problems in the detection of cardiac output from the temperature measurements with two thermal sensors, in that the measurements are critical to a fixed rate of injection of the cryogenic solution to provide steady state temperatures for analysis. This problem is recognized for example in an article of R. B. Dew entitled "Personal Computer System for Automatic Coronary Venous Flow Measurement" on pp 41-44 of the Proceedings, The Ninth Annual Symposium on Computer Applications in Medical Care, Nov. 10-13, 1985. Such a fixed injection rate is not readily attained in conventional catheters with cryogenic solutions administered manually in impulse fashion.
Introducible catheters are used not only for diagnostic data gathering, but also increasingly for performing therapeutic measures in the circulatory system. These invasive but nonoperative measures shall be explained with reference to the dilatation of a cardiac valve stenosis also described in the above-mentioned essay by Cribier et al. First a dilation catheter with a dilatation balloon is pushed beyond the narrowing, e.g. a stenosis of an aortic valve, the uninflated balloon being located in the stenosis area. With the aid of a channel leading to the proximal end within the catheter tube having a lumen before the dilatation balloon, the pressure before the stenosis, the prestenotic pressure, is now measured hydrostatically. Then, according to prior art, the blood pressure after the stenosis, the poststenotic pressure, is measured in the same way with the aid of a second arterial catheter.
Finally, a thermodilution catheter is introduced into the pulmonary artery, i.e., right heart and pulmonary circulation, via a vein. A predefined amount of a cryogen solution is then injected, as described above, into a proximal opening of this thermodilution catheter, which also has a distal temperature sensor, and the temperature gradient in time at the distal location of the temperature sensor is measured. The cardiac output is determined therefrom. The measured values determined with the individual catheters are fed separately to a computer. According to Gorlin's formula, the area KOF of a cardiac valve opening is directly proportional to the volume of blood V.sub.eff flowing through the valve per unit of time and inversely proportional to the root of the pressure gradient .sub.-- P.sub.m across the constricted valve: ##EQU1##
To calculate the extent of a stenosis or ascertain the success of distending the stenosis with the aid of so-called balloon dilatation, one must therefore not only determine the pre- and post stenotic pressure and thus the pressure gradient, but also the time intervals in which the blood flows through the narrowing as well as the amount of blood flowing through. This then can be used to determine the actual cardiac valve area. It does not suffice merely to measure the pressure gradient, since manipulations of arteries when introducing the catheter and the pressure on the blood vessel at the point of introduction often bring about a vegetatively induced change in the cardiac pumping amount, the heart rate and thus the cardiac output. Even with the same stenosis, different pressure gradients arise in accordance with the flowing amount of blood, so that as a rule several measurements must be taken to determine the cardiac output, generally two to three, in particular when the patient has a fluctuating cardiac output.
Only when reliable measured values are obtained is the dilatation balloon inflated in order to distend the stenosis.
The described process is time consuming. In particular when the measured values are being fed to the evaluating computer and during the evaluation time, all further activities are blocked. Evaluation generally takes two to four minutes. In particular the time factor is a risk for the patient. Thus, the inserted catheter may move out of its position or even fall out. There is also a danger of thrombo-embolism. Dysrhythmia can also occur which may be triggered, for example, directly by the mechanical contact of the catheter with the heart muscle. The necessary long dwell time of the catheter may also lead to a loss of blood at the point of introduction of the catheter, which again alters the cardiac pumping behavior. The introduction of a plurality of catheters is also very unpleasant for the patient. When the measured values are finally available, a measurement must be taken once again after dilatation of the stenosis in order to verify whether the stenosis has actually been distended to the desired extent.